1. Field of the Invention
The present invention relates to a radiation image converting material provided with a conductive polymer layer which is improved in the antistatic property. More particularly, the invention relates to a radiographic intensifying screen, and a radiation image storage panel employed in a radiation image recording and reproducing method utilizing a stimulable phosphor.
2. Description of the Prior Art
In a variety of radiography such as medical radiography for diagnosis and industrial radiography for non-destructive inspection, a radiographic intensifying screen is generally employed in close contact with one or both surfaces of a radiographic film such as an X-ray film for enhancing the radiographic speed of the system.
As a method replacing the radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In this method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The radiation image converting materials such as the radiographic intensifying screen employed in the conventional radiography and the radiation image storage panel employed in the above-described radiation image recording and reproducing method comprise a support and a phosphor layer provided thereon. Further, a transparent film is generally provided on the free surface of the phosphor layer (a surface not facing the support) to keep the phosphor layer from chemical deterioration and physical shock.
In the radiation image storage panel, the phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very advantageous for obtaining a visible image as described above, and the radiation image storage panel used in the method is desired to have high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.), as well as a radiographic intensifying screen used in the conventional radiography. In performing the radiation image recording and reproducing method, the radiation image storage panel is repeatedly used in a cyclic procedure comprising the steps of: exposing the panel to a radiation (recording radiation image thereon), irradiating the panel with stimulating rays (reading out the recorded radiation image therefrom) and irradiating the panel with a light for erasure (erasing the remaining radiation image therefrom). The panel is transferred from a step to the subsequent step in a transfer system in such a manner that the panel is sandwiched between transferring members (e.g., rolls and endless belt) of the system, and piled on other panels to be stored after one cycle is finished.
As a support material of the radiation image storage panel, desirably employed are plastic films such as a polyethylene terephthalate film and various papers from the viewpoint of flexibility required in the transferring procedure of the panel.
However, the panel is apt to be electrostatically charged on its surface in the repeated use comprising transferring and piling owing to the physical contact such as friction between the surface of the panel (surface of the phosphor layer or surface of the protective film) and a surface of other panel (surface of the support), friction between the edge of the panel and a surface of other panel, and a friction between the panel and transferring members (e.g., roll and belt). In more detail, the surface (front surface) of the panel made of a polymer material tends to be negatively charged and other surface (back surface) thereof tends to be positively charged. This static electrification causes various problems in the practical operation of the radiation image recording and reproducing method.
For example, when the surface of the panel is charged, the panel easily adheres to another panel and panes under adhesion panels are transferred together in layers from the piling position into the transfer system, whereby the subsequent procedure cannot be normally conducted. The read-out procedure of the panel is generally carried out by irradiating the panel with stimulating rays from the phosphor layer-side surface of the panel, and in this procedure, the charged surface of the panel is likely to be attached with dust in air, so that the stimulating rays are also scattered on the dust attached thereon and the quality of the resulting image lowers. Moreover, the resulting image provided by the panel suffers noise (static mark) when discharge of the panel takes place.
For improving the above-mentioned static electrification of the panel, there have been proposed various radiation image storage panels provided with antistatic functions, for example, a radiation image storage panel provided with an antistatic film comprising a conductive inorganic oxide on the surface of the protective film as described in U.S. patent application Ser. No. 818,239 (corresponding to EP Application No. 86100417.4) and a radiation image storage panel provided with an antistatic layer made of a conductive material and having a specific surface resistivity (10.sup.11 ohm) on the surface of the support not facing the phosphor layer or between the support and the phosphor layer as described in U.S. patent application Ser. No. 918,356 (corresponding to EP Application No. 86114224.8).
In the radiographic intensifying screen, the phosphor layer comprises a binder and phosphor particles dispersed. therein. When excited with a radiation such as X-rays having passed through an object, the phosphor particles emit light of high luminance (spontaneous emission) in proportion to the dose of the radiation. Accordingly, the radiographic film placed in close contact with the phosphor layer of the screen can be exposed sufficiently to form a radiation image of the object, even if the radiation is applied to the object at a relatively small dose.
The conventional radiography is generally conducted by encasing the radiographic intensifying screen and a radiographic film in a light-blocking cassette in such a manner that the screen and the film are arranged in close contact with each other. However, since both of the screen and the film are made of plastic material, the screen and the film are electrostatically charged due to contact with each other when the film is received in or is taken out of the cassette. As a result, discharge occasionally takes place, and an image formed on the film likely suffers noise (static mark), whereby accuracy of diagnostic examination lowers.
Recently, a continous radiographic system using no cassette (i.e., cassetteless system) has been developed and utilized for enhancing the examination efficiency. For example, in a radiographic apparatus for angiocardiography, a pair of radiographic intensifying screens fixed at the predetermined position, and in the radiographic operation, a number of radiographic films having been received in a magazine equipped in the apparatus are automatically and continuously transferred one after another to be received between the two screens. The used film is then transferred and received in a different magazine for used films by a transferring device, and at the same time an unused film is set between the screens. Thus, the radiographic procedures are continuously carried out at a high speed.
In the above-mentioned cassetteless system, the radiographic film is liable to be much more electostatically charged than the case of using the cassette, because contact of a film with another film and the contact of the film with the transferring members in the transferring procedure take place repeatedly, in addition to the contact of the film with the screen. As a result, a discharge phenomenon between the film and the screen takes place.
For preventing the occurrence of the static electrification and discharge, various technological measures have been proposed and practically utilized for the radiographic screen. For example, a method of coating or spraying a liquid antistatic agent onto the screen is generally utilized, but this method forms merely a coated layer on the surface of the screen. Hence, the coated layer tends to gradually separate from the screen as a lapse of time, owing to the contact with the radiographic film, etc., and the screen is reduced in the antistatic properties. Especially in the high speed radiography, there is such a trouble that the antistatic treatment (coating of the antistatic agent) should be repeatedly made at an appropriate interval because a great number of radiographic operations should be repeatedly performed.
For providing the antistatic properties to the radiographic intensifying screen, it is described that a carbon black layer is provided between the support and the phosphor layer and an antistatic agent is incorporated into its protective film in Japanese Patent Provisional Publication No. 52(1977)-28284. It is stated that according to this method the resulting radiographic intensifying screen can be prevented from electrostatical charging owing to the functions of the carbon black layer and the protective film.